1. Technical Field
This invention relates to an apparatus for interconnecting the detector assembly of an infrared system to the processor electronics of the system, and to a method for connecting the apparatus to the circuit pads of an infrared detector. More generally, the invention relates to an apparatus for interconnecting electronic components.
2. Discussion
A significant number of infrared detector/dewar applications require operation at cryogenic temperatures (&lt;135.degree. K). In order to minimize the thermal loads on the cooling devices of such systems, the detector assembly is typically packaged within a vacuum and mounted directly onto the coldfinger within a dewar. A thin film flexible cable is then used to electrically interconnect the cooled detector assembly with an ambient temperature input/output connector, thereby providing means for electrically connecting the cooled detector with external, ambient temperature processor electronics. Because the cable acts as a thermal short between the coldfinger and the ambient environment, thermal performance is the primary consideration in the design of the cable.
The two thermal characteristics of a cable that most heavily affect its thermal performance are the thermal conductance and I.sup.2 R heating of the conductors of the cable. Of these two, thermal conductance is usually the most critical. To minimize thermal conductance the conductor material is selected based on its electrical and thermal conductivity properties. Typical conductor materials like gold and copper exhibit the optimum electrical conductivity characteristics; however, the small cross-sectional area of the conductor, which is necessary so as not to exceed the thermal conductance requirements, is far below industry processing limits. Consequently, constantan, nickel or stainless steel have become the metal-foils of choice because of their low thermal conductivity properties. But even with these metals, the conductor lengths required to achieve a sufficiently large thermal resistance are many times longer than that required to interconnect the detector assembly and the input/output connector.
It would be desirable to have a thin film cable whose conductors are made of gold. Such a cable would provide optimum thermal performance in connecting the cooled detector assembly of an infrared array with the ambient temperature input/output connector of the dewar.
An additional problem has been inadequate bond or adhesive strength between the metal-foil conductors and the dielectric substrates adhered to each side of the conductors. This problem has been particularly, acute with respect to thin film cables used to connect to an infrared detector assembly, operating at cryogenic temperatures.
Other disadvantages exist. Prior art metal-foil conductors, being laminated to dielectric substrates on each side of the metal-foil, generally require the use of material that can contaminate the dewar environment resulting in reduced vacuum life and lengthy pump-down times. Metal-foil cables also require complex multi-operation manufacturing processes that are low yield and result in a high unit cost. In addition, laminated cables are not very flexible and continued working or bending of them during the assembly process often results in separation of the foil and dielectrics. Moreover, the non-flexible nature of laminated metal-foil cables typically requires that each end of the cable be attached to a part of the dewar structure to provide a strain relief for the fine wire interconnections and to protect them from damage during thermocycling of the dewar. It would therefore be advantageous to have a thin film cable of simple construction that avoids the use of adhesives and the like in the manufacturing process, which is more flexible than a laminated metal-foil cable without its conductive elements being prone to separation from its substrate material, and which operates reliably in a cryogenic environment.
There are two major prior art methods of connecting thin film cables to the circuit pads of infrared detectors and other electronic components. The first consists of attaching a metal-foil cable to a part of a mounting structure at each end of the cable with adhesives or mechanical clamps and interconnecting the cable to the circuit pads of a detector or other electronic component by wirebonding fine wire to the circuit pads and the conductors of the cable. The second uses a direct fine wire connection from a circuit pad to a second connection point.
A problem exists in both prior art methods in that completion of the interconnection is dependent on fine wire interconnections achieved by wire bonding. These wirebonds are frail, easily broken, and require two joints in a circuit line (one at either end of the wire).
A third prior art method of connection, Tape Automated Bonding (TAB), eliminates the need for wirebonding by direct bonding of the conductor to the circuit pads of the detector or other electronic device. However, with TAB, substrate material is usually removed around the conductor by toxic agents or the like so that a bonding tool can make actual contact with the conductor material. This extra step is costly and inconvenient.
It would therefore also be desirable to have a method of connecting a thin, film cable to the connector pads of an infrared detector or other electronic device that eliminates the need for the fine wire connections associated with wirebonding or the need to remove a portion of the substrate material required by the TAB connection method.